In many medical, biological and chemical analytical applications, it is important to determine characteristics of fluids taken from a sample under study or consideration. For example, during hemodialysis to treat blood from a patient suffering kidney failure and related conditions, it is important to analyze the blood and/or dialysis fluid to determine the effectiveness of the treatment. Additionally, medical professionals may use information about the purity of the patient's blood being treated to form medical opinions and propose further therapy. Many various technologies have been developed for conducting such fluid analysis.
One such technology utilizes the electrical properties of the fluid under study to determine its purity and other data. All fluids have some measurable ability to conduct electricity and the purity or composition of the fluid may affect its conductivity. Hence, conductive sensing may be used to reveal information about the chemical composition of the fluid. Connecting a sample of the fluid into an appropriately designed electrical circuit enables analysis of the fluid's conductive properties and thus determination of it purity or composition and the effectiveness of dialysis filtration.
Conductivity analysis of fluids such as dialysis treated blood presents many technical challenges and difficulties. For example, while a sample of the fluid may be tested in isolation, it is often preferable to analyze the fluid in process. To accomplish this, it is necessary to incorporate the test circuit into the process such as making the test circuit part of the hemodialysis machine or system. Another complex issue involves the actual electrical coupling between the electrical circuit and the fluid being analyzed. For example, electrodes can be disposed into a channel through which the test fluid is directed. This design, however, may lead to fouling and contamination of the electrodes by the test fluid. Conversely, and especially when the test circuit is reused on multiple occasions, the electrodes may contaminate the fluid under test with traces of previously tested fluids.
To address contamination problems, various contactless designs for conductivity testing circuits have been designed and incorporated in fluid analysis systems. One such contactless design utilizes the principles of capacitive coupling between the electrode and the fluid so that the two do not have to be in direct physical contact. Applying an alternating current to an electrode placed proximate to a channel or test cell containing the fluid of interest will cause the electrode to capacitively couple with the fluid and enable gathering of electrical data regarding the fluid. Capacitively-coupled contactless conductivity detection (C4D) detectors are known and described in the prior art such as, for example, in International Publication No. WO 2010/016807 and U.S. Pat. No. 7,629,797. The sensitivity and accuracy of such detectors may be affected by the impedance and/or reactance associated with the circuit or system elements, the geometry and design of the electrodes and the test cell, and the material properties of the test fluid and the circuit or system elements. It is therefore necessary for a conductivity detector design to account these and other considerations to improve sensitivity and accuracy.